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Final Cruise Report- Arabian Sea Process Cruise 6

We are returning to the Gulf of Oman, having finished our last stations of process cruise 6 to the Arabian Sea. The JGOFS station grid was completed last week with a final exciting Undulating Oceanographic Recorder (UOR) tow into shallow shelf waters. A cold filament ran offshore, through several of the stations of the JGOFS line, affecting them greatly in terms of their hydrography and bio-optical properties. From the last JGOFS station, we studied this cold filament further, which had curved well to the southwest over the previous 4-5 days. We visited a series of stations, along-shore, from the cold tip of the filament, across adjoining warm water (likely of shelf origin) and ultimately back into the same filament again at the JGOFS line. We almost lost the filament because of 4 days of poor satellite coverage due to clouds, but with the UOR data, shipboard continuous underway data, and a little luck, we managed to find the original filament again. Interestingly, with most of these features, warm waters of shelf origin had higher productivity and biomass than the cool waters. Moreover, nitrate has been absent in cool surface waters, and up to 1.5uM in warmer waters advected off the shelf. There has been considerable discussion on the cause of this pattern. Typically, 20uM nitrate water has been found at the 18oC to 22oC isotherm. Prochlorophytes have been observed in these warm shelf waters and the phytoplankton assemblage were low diversity. Coccolithophores have been quite abundant, accounting for 15-30% of backscattered light (typical values with peaks up to 60%). Unfortunately, we have observed no Trichodesmium in microscope samples, nor any surface slicks caused by this blue-green algae, but we have observed a multitude of other algal taxa for bio-optical studies. The next feature work was in a warm patch east of the JGOFS line, densely populated by cyanobacteria and coccolithophores. The patch was only about 1000 square kilometers but it impressed all with its high nitrate, productivity, and even the presence of sperm whales. Next, we steamed into a 3000 km2 cyclonic cold-core eddy 350 kilometers offshore; this feature was chosen as it had a temperature gradient steeper than any other observable from the AVHRR satellite imagery. It proved to be one of the highlights of the cruise, as it turned out to be a major bloom of two varieties of cyanobacteria (based on fluorescence properties and flow cytometry) mixed with some coccolithophores and a few naked dinoflagellate species. We knew we had hit something impressive when all of the underway optical sensors saturated at the frontal boundary. Numerous tintinnid lorica in the samples suggested protozoan grazing of the cyanobacteria. The 1% light depth (PAR) in this eddy was reduced from about 55m outside to 16m at eddy center. Chlorophyll concentrations were as high as 1.8mg/m3, and absorption values were three times higher than any other offshore station that we have visited. Total organic carbon reached a high for the cruise at 110uM C. The oxygen minimum was extremely shallow in this eddy; oxygen was 90% depleted by 25m. Productivity samples are still being worked up. On our return transit, we stopped on the warm side of a front, located 165 kilometers off of Masirah and again found large amounts of coccolith calcite in the water. We revisited our earlier JGOFS station off of R'as Al Hadd, enroute to the Gulf of Oman, and are currently towing the UOR one more time across the R'as Al Hadd front, to the center of a warm core eddy that we visited 27 days ago. This feature was populated by Synechococcus and Prochlorococcus. Subsequent pigment analysis has indicated the presence of significant numbers of cryptophytes which we are anxious to re- sample. The final tally for our sampling on Process 6 is as follows: UOR towed 3120km (effectively providing a CTD and optics cast in the top 100m, every 3 kilometers of the cruise track) and the trace metal sampler was deployed for the same distance. Both UOR and trace metal sampler were deployed and recovered 27 times. Trace metal and DOC teams worked extremely well together. Underway discrete samples were taken every 2 hours of transit time (for primary production, calcification, nutrients, salinities, pigments, DOC/DON, trace metals, and cell counts). Twenty eight stations were occupied, with a total of 178 casts with the 24 bottle rosette: 52 hydrocasts, 10 intermediate casts, 15 deep casts, 25 casts for simulated in situ primary production and calcification (and 8 successful deployments of the in situ array), 57 CTD casts for diel experiments and nitrogen production experiments, and 19 casts for P vs E measurements. In addition, there were 39 optics casts with the MER spectoradiometer and 12 vertical optics casts with the UOR. Between these casts, tests were being run with an INMARSAT Standard-B High Speed Data system as the ship changed headings.This is one of the first B systems to be commissioned for maritime use. Transfer of satellite images was achieved at 48 kilobits/s, and the first links of the Thompson to the WWW browser were made. The scientific party worked extremely hard, finishing most stations well ahead of schedule. Moreover, Capt. Gomes, the crew, and marine techs aboard the Thompson are to be commended for making this a wonderful expedition. We have learned much about the bio-optical variability, standing stocks and production rates of the various carbon pools in the Gulf of Oman and Arabian Sea. William Balch Bigelow Laboratory for Ocean Science Chief Scientist- Process Cruise #6